In growing plants indoors and in containers such as pots, problems are encountered which are not present when the same plants are grown outdoors. Due to the unnatural environment of a relatively small container and indoor conditions, various problems arise. For example, various salts such as sodium carbonate, calcium carbonate and sodium chloride present in the water used to water the plants eventually is deposited on the sides of the container and on the surface of the soil. Under normal conditions outdoors, this salt would be sequestered through the soil, enriching the same.
Also, the soil in the pot becomes crustly and contracts from the sides of the pot. The aeration is restricted and the soil's moisture holding capacity is reduced. This can lead to root binding, as the roots try to reach whatever moisture is available in the drainage area on inside surfaces of the pot but farthest from the center root cluster. The problem is not solved by the simple addition of normal fertilizers. While these supply certain necessary nutrients to the plants, they do not result in the soil becoming more moist and porous and capable of handling the salts introduced by repeated watering. In fact, some of these fertilizers introduce salts of their own which may also be deposited on the pot and on the surface of the soil.
Part of the problem is that the normal symbiotic relationship that exists is destroyed. In the natural cycle, old plant material of various kinds is broken down by microorganisms present in the soil and this results in carbohydrates being formed. The carbohydrates provide the energy source for the microorganisms to build protein. The growth of the microorganisms and enzymes that normally takes place creates a very large cation exchange capacity for the storage of nutrients for both the plant and microorganisms. Where the plant is in a pot, the amount of old plant material is very limited and sooner or later the microorganisms and enzymes will gradually disappear due to lack of nutrient material for the microorganisms. The result will be that this normal symbiotic relationship is broken down and the soil ceases to become "living" soil.
The result is that, in many cases, after a potted plant becomes so old, it is discarded and replaced by a new plant, despite the fact that the plant itself is often perfectly capable of further growth if the soil were in proper condition.